diff --git a/implementations/javascript/package.json b/implementations/javascript/package.json
index 5e4b6c8..38a0fe2 100644
--- a/implementations/javascript/package.json
+++ b/implementations/javascript/package.json
@@ -1,6 +1,6 @@
 {
   "name": "preserves",
-  "version": "0.6.3",
+  "version": "0.6.4",
   "description": "Experimental data serialization format",
   "homepage": "https://gitlab.com/preserves/preserves",
   "license": "Apache-2.0",
diff --git a/implementations/javascript/src/decoder.ts b/implementations/javascript/src/decoder.ts
index a730c50..59f5303 100644
--- a/implementations/javascript/src/decoder.ts
+++ b/implementations/javascript/src/decoder.ts
@@ -28,7 +28,11 @@ export class Decoder<T extends object> {
     }
 
     write(data: BytesLike) {
-        this.packet = Bytes.concat([this.packet.slice(this.index), data])._view;
+        if (this.index === this.packet.length) {
+            this.packet = underlying(data);
+        } else {
+            this.packet = Bytes.concat([this.packet.slice(this.index), data])._view;
+        }
         this.index = 0;
     }
 
@@ -142,6 +146,10 @@ export class Decoder<T extends object> {
 
     try_next(): Value<T> | undefined {
         const start = this.index;
+
+        if (start >= this.packet.length) return void 0;
+        // ^ important somewhat-common case optimization - avoid the exception
+
         try {
             return this.next();
         } catch (e) {